1. Field of the Invention
The present invention relates to a battery voltage detecting circuit.
2. Description of the Related Art
In an apparatus such as a notebook-sized personal computer that uses rechargeable batteries, the voltage of each of the batteries needs to be highly precisely detected to manage recharge/discharge of each of the batteries that are connected in series. FIG. 3 depicts a common configuration of a battery voltage detecting circuit (see, for example, Japanese Patent Application Laid-Open Publication No. 2002-243771). A battery voltage detecting circuit 100 is used for detecting the voltages of four batteries BV1 to BV4 that are connected in series, and is configured including an operational amplifier 110, resistors R1 to R4, switches SW0M to SW4M and SW0P to SW3P, and a power source 115 that outputs a reference voltage VREF. In such a battery voltage detecting circuit 100, when a voltage VBV4 of the battery BV4 is to be detected, the switches SW4M and SW3P are turned on and other switches are turned off. Thereby, a voltage VOUT that corresponds to the difference between a voltage V4 of a plus terminal of the battery BV4 and a voltage V3 of a minus terminal thereof is output from the operational amplifier 110 to an AD converter (ADC) 120. The ADC 120 converts the voltage VOUT into a digital value and, thereby, the voltage VBV4 of the battery BV4 can be detected. Similarly, the switches SW3M and SW2P are turned on and other switches are turned off and, thereby, a voltage VBV3 of the battery BV3 can be detected. The switches SW2M and SW1P are turned on and other switches are turned off and, thereby, a voltage VBV2 of the battery BV2 can be detected. Furthermore, the switches SW1M and SW0P are turned on and other switches are turned off and, thereby, a voltage VBV1 of the battery BV1 can be detected.
When lithium ion batteries are used as the batteries BV1 to BV4, each of the voltages VBV1 to VBV4 between both terminals of each of the batteries BV1 to BV4 reaches about 4.5 V when the batteries are fully recharged. Assuming that each of the voltages VBV1 to VBV4 respectively of the batteries BV1 to BV4 is defined as 5 V taking the design allowance into account, the batteries BV1 to BV4 connected in series as a whole generate a voltage of 20 V and, therefore, the battery voltage detecting circuit 100 needs to be of a high-voltage type. On the other hand, a circuit including the ADC 120 in a control system generally uses a power source voltage of about 3 V and the voltage VOUT output from the battery voltage detecting circuit 100 needs to be 3.3 V or lower.
In this case, assuming that the resistance values of the resistors R3 and R4 respectively are R3 and R4, the gain GAMP of the operational amplifier 110 is R4/R3. Therefore, the VOUT output when the voltage VBV4 of the battery BV4 is detected is VOUT=VBV4/GAMP+VREF=(V4−V3)R3/R4+VREF. Assuming that VBV4 is 5 V and VREF is 0.2 V, a condition for the gain GAMP of the operational amplifier 110 to be VOUT≦3.3 V is that GAMP≦(VOUT−VREF)/VBV4=(3.3−0.2)/5≈0.6. Based on this condition, the resistance values respectively of the resistors R3 and R4 are selected such that the gain GAMP of the operational amplifier 110 becomes about 0.6 and, thereby, the voltage VOUT output to the ADC 120 can be made 3.3 V or lower. However, in this case, the operational amplifier 110 needs to be of a high-voltage type and this results in increase of the cost of the battery voltage detecting circuit 100.
To make the operational amplifier 110 need not to be of a high-voltage type, the voltage applied to the operational amplifier 110 needs to be 3.3 V or lower. That is, to make the voltage V+ applied to a + input terminal of the operational amplifier 110 be 3.3 V or lower, that (V3−VREF)R4/(R3+R4)+VREF≦3.3 needs to be satisfied. This leads to R4/(R3+R4)≦(3.3−VREF)/(V3−VREF)=(3.3−0.2)/(15−0.2)=3.1/14.8≈0.21. Therefore, the gain GAMP of the operational amplifier 110 is GAMP=R4/R3≦0.21/(1−0.21)≈0.26. Therefore, by selecting the resistance values respectively of the resistors R3 and R4 such that the gain GAMP of the operational amplifier 110 is about 0.26, the operational amplifier 110 can be made need to not be of a high-voltage type. However, in this case, the voltage VOUT input into the ADC 120 goes low because the gain GAMP of the operational amplifier 110 is small. Therefore, to highly precisely detect the battery voltage, the ADC 120 needs to be of a high-precision type and this results in increase of the cost thereof.
In the battery voltage detecting circuit 100, when the voltages respectively of the batteries BV1 to BV4 are detected, a current flows through the resistors R1 and R3 that are connected to the input terminal of the operational amplifier 110. Therefore, to suppress the discharge of the batteries BV1 to BV4 caused by this current, high resistances such as about several mega ohms each need to be used as the resistances respectively of the resistors R1 and R3. To highly precisely detect the voltages respectively of the batteries BV1 to BV4, the resistors R1 to R4 need to be those that each have a low voltage-dependent resistance value. When an integrated circuit using resistors that each have a high resistance value with low voltage dependency as above is manufactured, special process steps need to be provided and this results in increase of the cost thereof.